High-speed, high-power electric motors that operate at variable speed are increasingly utilized in a range of industrial, mining and drilling activities. Further, the activities often depend upon a high-degree of reliability. In operations such as crude oil pumping from remote global locations where access to pumping stations is difficult and time-consuming, reliability of motor operation may prevent dangerous, costly and extended outages. Simple, sturdy and reliable power converters are preferred for such high-speed, high-power motor operations. It is well known that providing multiple individual components, such as series or parallel semiconductor switches, may increase the likelihood that any one individual component switch may randomly fail. Added elements such as snubber circuits for semiconductor switches, further increases the number of components that can fail. It is desirable to arrange the power converter in a simple configuration, with as low a part component count as is possible. However, individual components, such as the semiconductor switches for the power converter, can be operated with satisfactory margin to thermal and other functional limits to avoid failures in the simplified configuration.
A power converter used in one example embodiment may be a simplified three-phase, wye-connected H-bridge converter configuration, such as is illustrated in FIG. 1. Each phase of the converter includes a power source/sink 20 with a de power shaping circuit, represented by capacitor 30. The power source/sink/20 and dc power shaping circuit, represented by capacitor 30, establish a dc-link voltage input to the semiconductor switches of the bridge. Insulated-gate bipolar transistors (IGBTs) 40 with built-in diodes 45 may form each leg of the H-bridges 50, for example, but other power semiconductor switches such as integrated-gate commutated thyristors (IGCTs) or metal-oxide semiconductor field-effect transistors (MOSFETs) could be used instead. The type of power semiconductor switch is not important to the analysis. Each H-bridge includes two legs, an output leg 60 and a neutral leg 65. Each phase output, phase A 70, phase B 75 and phase C 80 is connected to the midpoint 85 of the respective output bridge leg 60. Each neutral connection to wye-point 90 is tied to the midpoint 95 of the respective neutral output leg 65.
Gating controls 35 provide control signals 36, 37, 38 for switching semiconductor switches 40 of Phases A, B, and C of the H-bridge converter, according to predetermined switching patterns. Gating controls may provide for synchronous switching or asynchronous (pulse-width modulation, for example) switching of the semiconductors switches 40 of the H-bridge.
However, to improve availability of operation of the motor loads, it is desirable to further balance temperatures, reduce switching and power losses and harmonic distortion. Reduction in switching loss will keep semiconductor H-bridge switches operating at lower temperatures with a greater margin to failure.
Accordingly, there exists a need for systems and methods for controlling a converter for powering a load in order to balance temperatures, and to reduce switching loss and total harmonic distortion.